The present invention relates to a measuring probe for potentiometric measurements. The probe has a housing of an electrically insulating material surrounding an enclosed space that contains a primary reference element and an electrolyte. The housing has at least one opening through which the electrolyte can be brought into contact with a sample solution on which a measurement is to be performed. The enclosed space is filled with an ion-permeable, micro-porous, high-viscosity polymer substance which, in combination with the electrolyte, forms a filler mass of the measuring probe. The invention also relates to a method of monitoring the state of aging of the measuring probe, and it further relates to the use of the measuring probe for applications in process monitoring and process control.
A widely used kind of measuring probe for potentiometric measurements of ion concentrations or redox potentials is equipped with a diaphragm of porous material. The diaphragm serves to bring a referencexe2x80x94and/or bridge electrolyte, normally in the form of a liquid contained inside the measuring probe, into contact with a test solution. Particularly in chemical or micro-biological process-monitoring and process-control applications, the diaphragm may be subject to contamination which can falsify the results of the measurements.
Another measuring probe, known from DE 34 05 431 C2, belongs to the same general category of measuring probes but does not have a diaphragm and is significantly less prone to contamination. It has a housing of an electrically insulating material with at least one enclosed space containing a reference element and an electrolyte. The housing has at least one opening through which the electrolyte can be brought into contact with the test medium on the outside of the housing, i.e., a liquid solution on which a measurement is to be performed. The enclosed space inside the housing is filled with an ion-permeable, micro-porous, high-viscosity polymer substance which, in combination with the electrolyte, forms a filler mass of the measuring probe. This type of construction assures that the electrical potential measured at the reference element is highly constant even if the solutions being measured are strongly contaminated. In addition, the measuring probe can sustain pressure levels significantly in excess of 10 bar.
Measuring probes of the foregoing description are known to have the problem that as the cumulative operating time of the probe advances, the electrolyte that is initially contained in the polymer substance will to an increasing degree migrate into the test solution, resulting in a progressively spreading electrolyte deficiency in the polymer substance inside the housing. The increasing electrolyte deficiency in the polymer substance is also referred to as the aging process of the measuring probe and produces the undesirable effect that, when the electrolyte deficiency eventually reaches the reference element, there will be a change in the electrical potential measured at the reference element. To avoid the risk of erroneous measuring results, it is therefore necessary to monitor the aging process of the measuring probe. In particular, it should be possible to detect sufficiently in advance, i.e., with an adequate pre-warning interval, when the electrolyte deficiency is approaching the reference element.
According to DE 34 05 431 C2, the problem of detecting the advancement of the electrolyte deficiency can be solved by using an electrolyte consisting of a suspension of homogeneously distributed particles of a neutral salt with ions of equal transport number in an aqueous solution of the neutral salt. The polymer substance and the neutral salt suspension together form a gel that has a turbid appearance due to the salt particles in suspension. The state of aging of the measuring probe can be visually detected, as the turbidity disappears progressively with the advancement of the aging process. The reason for the decrease in turbidity is that the suspended neutral salt particles continuously pass into solution until a final state has been reached where there are essentially no suspended particles left and, as a result, the turbidity is strongly diminished. It has been found that in the aging process, a clearly visible boundary develops between a turbid portion of the gel where the neutral salt particles are homogeneously suspended and a comparatively clear portion where the neutral salt particles have passed into solution. As the boundary advances over time from the opening in the housing towards the reference element, the state as well as the speed of aging of the measuring probe can be determined from a visual observation of the boundary in the polymer gel.
However, the measuring probe according to DE 34 05 431 C2 has several drawbacks. To monitor the state of aging, it is necessary to be able to clearly see inside the enclosed space of the measuring probe. This precludes the use of a non-transparent material for the housing, and it also presents a problem with a transparent housing if the latter becomes contaminated by surface deposits. A further severe problem occurs if the gel in the enclosed space becomes discolored or contaminated, e.g., by the infusion of colored substances or infiltration of dirt particles from the test solution, which could make it practically impossible to visually detect the boundary of the electrolyte deficiency. It also has to be counted as a drawback that the electrolyte needs to be a suspension of homogeneously distributed particles of a neutral salt with ions of equal transport number in an aqueous solution of the neutral salt, a condition that excludes other kinds of electrolytes from being used in the measuring probe.
A first object of the present invention is to provide an improved measuring probe that is free of the aforementioned drawbacks. A further object of the invention is to provide a method of monitoring the state of aging of a measuring probe, and a third object is to propose a use of the improved measuring probe.
A measuring probe according to the present invention has a housing of an electrically insulating material surrounding an enclosed space that contains a primary reference element and an electrolyte. The housing has at least one opening through which the electrolyte can be brought into contact with the outside of the housing, i.e., with a sample solution on which a measurement is to be performed. The enclosed space is filled with an ion-permeable, micro-porous, high-viscosity polymer substance which, in combination with the electrolyte, forms a filler mass of the measuring probe. The measuring probe of the present invention has a secondary reference element contained inside the enclosed space and arranged in such a manner that an electrolyte deficiency advancing from the opening towards the primary reference element arrives at the secondary reference element before it reaches the primary reference element.
In the measuring probe according to the foregoing description, the state of aging is monitored by using a method that is likewise part of the present invention, with the steps of
a) detecting continually or at intervals the difference (V1xe2x88x92V2) between the respective electrical potentials V1 and V2 of the primary and secondary reference element, and
b) generating a signal when the difference between the potentials exceeds a previously specified threshold value, and/or if the difference between the potentials changes at a rate that exceeds a previously specified threshold rate.
The invention also encompasses the use of the inventive measuring probe in process-monitoring and process-control applications.
With the inventive concept of arranging a secondary reference element in the enclosed space of the measuring probe so that an electrolyte deficiency advancing from the opening towards the primary reference element reaches the secondary reference element before it arrives at the primary reference element, it is no longer necessary to visually inspect the enclosed space in order to determine the state of aging of the measuring probe. In particular, a measuring probe according to the invention can have a non-transparent housing, or it can be a built-in part of an apparatus. In addition, the state of aging can also be monitored if the housing is covered with contaminants, as may be the case if the measuring probe is used for dirty or foamy sample solutions. Furthermore, a measuring probe according to the present invention does not require the special kind of electrolyte that is needed with the state-of-the-art probe described above in order to make the boundary of the electrolyte deficiency visible. Thus, there is a wider choice of electrolytes available to be used in combination with the polymer substance, without the previous limitation to a suspension of homogeneously distributed particles of a neutral salt with ions of equal transport number in an aqueous solution of the neutral salt. In particular, a measuring probe of the present invention can work with an electrolyte consisting of a saturated or near-saturated solution of a neutral salt in an appropriate solvent, e.g., a near-saturated solution of potassium chloride in water.
With the inventive method of continuously or intermittently monitoring the difference between the electrical potentials of a primary reference element and a secondary reference element against a threshold value or a threshold rate of change, the state of aging of the measuring probe can be reliably ascertained. In particular, the invention provides a convenient way of automating the monitoring function. Thus, the inventive measuring probe can be used advantageously for applications in process monitoring and process control.
In a preferred embodiment of the invention, the primary reference element is configured as a primary cartridge containing a primary electrode and a primary electrolyte. The required electrical contact between the primary electrolyte and the electrolyte in the enclosed space inside the housing of the probe can be established in a known manner through a diffusion zone of, e.g., fiberglass wool or cotton. Alternatively, the primary cartridge may have a primary passage opening. The choice of the primary electrolyte generally depends on the design of the primary electrode, but also on the composition and properties of the electrolyte in the enclosed space. In one embodiment of the invention, the primary electrolyte is mixed with the same polymer substance that is also used in the filler mass.
In advantageous embodiments of the invention, the secondary reference element is configured as a secondary cartridge containing a secondary electrode and a secondary electrolyte. Analogous to the primary cartridge, the secondary cartridge can have a secondary passage opening, and the secondary electrolyte can be provided in a mixture with the same polymer substance that is also used in the filler mass.
In an advantageous embodiment of the invention, one end of the secondary electrode is immersed in the primary electrolyte. In another embodiment, one end of the secondary electrode is immersed in the electrolyte in the enclosed space of the measuring probe in an area outside the primary cartridge. These two arrangements of the secondary electrode have the advantage that they lengthen the distance along which the boundary of the electrolyte deficiency advances from the probe opening to the primary reference element, so that the measuring probe lasts longer before it reaches the end of the aging process.
The primary electrode and/or the secondary electrode can be configured as wire electrodes of a known design, e.g., in the form of a silver wire, one end of which is coated with silver chloride and immersed in the electrolyte of the respective electrode. Alternatively, at least one of the reference elements may have an electrode in the form of a conductive track deposited, e.g., on the inner or outer wall surface of a reference element cartridge or on the inner wall surface of the measuring probe housing.
In a particularly preferred embodiment of the invention, the electrolyte in the enclosed space of the measuring probe housing is a suspension of homogeneously distributed particles of a neutral salt with ions of equal transport number in an aqueous solution of the neutral salt. This has the advantage that the entire filler mass contains a considerable supply of electrolyte, which works to the advantage of a long operating period of the measuring probe. Potassium chloride has proven to be a particularly advantageous choice for the neutral salt. Preferred is a suspension of fine particles of potassium chloride in an aqueous or part-aqueous potassium chloride solution. The proportion of suspended potassium chloride should be at least 30% and may be as high as 1500% in relation to the dry weight of the polymer substance. A preferred range is between 100% and 800%, and the highest preference is for 200% to 400%.
According to a further embodiment of the invention, the measuring probe is configured as a reference electrode, so that the measuring probe can be used as a reference element, e.g., for a pH electrode or another measuring electrode. In another preferred embodiment of the invention, the measuring probe is configured as a single-rod measuring chain, which has the advantages that it is compact and simple to operate.
A preferred measuring probe according to the invention is also equipped with a means for monitoring the difference between the respective electrical potentials of the primary and secondary reference element. This makes the measuring probe particularly suitable for applications in process monitoring and/or process control.